Understanding the effects of heat stress on yield and grain quality component traits in rice

Higher ambient temperatures during rice cropping season reduce grain yield and quality, yet maturity group-specific responses to heat stress are largely unknown. A field trials of 36 rice varieties were conducted in Arkansas, United States, using two planting dates in a randomized complete block design with four replications under season-long conventional flood irrigation. Varieties were classified into early (n = 8), medium (n = 13), and late maturing (n = 15) based on heading dates. Cumulative heat exposure was quantified by using heat degree days (HDD) within key developmental stages. Heat stress effects were most pronounced during nighttime, suggesting that the observed yield and quality alterations were largely attributable to high nighttime temperatures (HNT). Early-maturing varieties showed greater tolerance to HNT, as evidenced by a non-significant decline in grain yield as the HDD increased. In contrast, medium- and late-maturing varieties exhibited significant yield losses per unit HDD (medium: -0.368 g plant^-1 per °C·day; ∼0.9% per HDD; late: -1.024 g plant^-1 per °C·day; ∼1.4% per HDD). Grain traits varied among maturity groups, with medium- and late-maturing varieties producing thinner and chalkier grains, while early-maturing varieties largely maintained grain weight. Across traits, variety and maturity group explained most variation, and significant variety × planting date interactions highlighted exploitable genotype-by-environment (G × E) effects on nighttime heat tolerance. Early-maturing varieties, including Norin 20, ZAO 402, and Geumobyeo, demonstrated strong resilience to HNT stress. Planting dates adjustments may, therefore, offer a practical adaptation strategy to mitigate heat stress under warming cropping seasons, although further investigation is needed.